This invention relates generally to a high pressure sodium vapor lamp construction for operation on sonic frequency pulses and more particularly to an improved structural configuration in said type lamp enabling operation without excessive acoustic noise.
High pressure sodium vapor lamps are now well known and widely used for street, roadway and area lighting applications. The basic lamp type is described in U.S. Pat. No. 3,248,590 issued to Schmidt in 1966 and generally comprises an outer vitreous envelope or jacket of glass within which is mounted a slender tubular ceramic arc tube. The ceramic envelope is made of a light transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises sodium along with a rare gas to facilitate starting, and mercury is generally included for improved operating efficiency. The ends of the alumina tube are sealed by suitable closure members affording connection to the electrodes. The outer envelope is generally provided at one end with a screw base having shell and eyelet terminals to which electrodes of the arc tube are connected. The original high pressure sodium vapor lamps were conventionally operated on 60 cycle alternating current by means of ballasts to limit the current to that of the lamp rating. In such operation, the light generated by the discharge is due almost exclusively to the excitation of the sodium atom through the self-reversal and broadening of the sodium D lines at 590 nanometers. The lamp efficiency is high when operated in such manner, up to 130 lumens per watt depending upon lamp size but the color temperature is low from approximately 1900.degree. to 2100.degree. Kelvin. While colors of the objects being illuminated in all portions of the spectrum are recognizable, those at the "cool" end such as violets, blues and to some extent greens are muted or grayed down. As such, the lamps were not found particularly acceptable for indoor applications where critical color discrimination is required. More recently, however, the color temperature of high pressure sodium vapor lamps has been raised and their color rendition has been improved by employing pulse operation. The principle of such operation is described in U.S. Pat. No. 4,137,484 issued to Osteen and assigned to the assignee of the present invention. By utilizing pulse repetition rates in the sonic ranges from about 500 to 2000 Hertz and short duty cycles from about 10 to 30 percent, the color temperature has been increased from a common value of 2050.degree. K. to as high as 2700.degree. K. with substantially no reduction in the lamp efficacy, or even higher than 2700.degree. K. at the price of some reduction in efficacy.
In still another U.S. Pat. No. 4,061,939 issued to Strok and also assigned to the present assignee there is disclosed a jacketed high pressure sodium vapor lamp construction which avoids the undesirable acoustic noise accompanying such sonic frequency pulse operation. More particularly, it is recognized therein that a troublesome audible noise problem is encountered at the pulse operating frequency since the ear is sensitive to an audio range extending from about 16 up to about 20,000 Hertz. Also recognized is that the noise problem is aggravated by the short duty cycle being employed which means an abrupt rise and fall in current at every pulse inducing higher frequency harmonics which may be even more penetrating to the human ear. To effect a noise reduction when such lamps are being operated in this manner, only non-magnetostrictive metals are employed for the major lamp component parts. For example, the ceramic arc tube is said to be constructed with electrode supporting end closures fabricated with non-magnetostrictive metals such as niobium or tantalum. Likewise, a conventional nickel-iron metal frame supporting said ceramic arc tube was found to be another noise source so that non-magnetostrictive titanium metal was substituted in the construction of said lamp parts. A still further reduction of magnetostrictive metals in said lamp construction is also therein disclosed whereby nickel wire inleads are replaced with titanium and copper conductors. Since titanium is further recognized therein to serve as a gettering agent, the customary practice of incorporating ring getter elements in the lamp construction can also be avoided.
Unfortunately a number of the above listed non-magnetostrictive metals for use as components in this type lamp construction are both scarce and expensive commodity items. Thus, titanium is expensive and the effective use of this and other bulk metal getters in the lamp construction frequently requires a careful vacuum heat treatment for outgassing. Ring getter elements employing a vaporizable barium substance, on the other hand, remain a most effective gettering agent for the common impurities found in lamp constructions including water vapor, oxygen, carbon dioxide and nitrogen. Typically, a barium material further containing aluminum and packed within a small circular metal channel ring emits a directional beam of barium atoms when it is simply subjected to dull red heating by a radio frequency induction coil and with the flashed material thereafter providing effective gettering action. It remains desirable, therefore, to retain use of such ring getter elements in this type lamp construction while still not subjecting the lamp to excessive acoustic noise.
Accordingly, one object of the present invention is to provide means for utilization of ring getter elements in a high pressure sodium vapor lamp being operated by sonic pulses while not causing said lamp to encounter excessive acoustic noise.
Another object of the present invention is to provide a particular combination of at least one ring getter element with other non-magnetostrictive structural components in this type lamp so as to achieve low noise sonic pulse operation.
Still another object of the present invention is to provide means whereby at least one ring getter element is physically positioned in this type lamp so as to avoid acoustic coupling during lamp operation.
These and other objects of the present invention will become more apparent from the more detailed description hereinafter provided.